Semiconductor devices having a superlattice structure in which there is a periodic variation in semiconductor composition are described in Esaki et al. U.S. Pat. Nos. 3,626,257 and 3,626,328, issued Dec. 7, 1971. These early superlattice devices comprised a one dimensional transport device formed by either doping or alloying techniques. Both methods of fabricating a superlattice structure involves epitaxially growing a semiconductor material which is periodically doped so as to produce alternating multi-thin layers having different band energies. These ultra-thin, well defined multi-layered semiconductor structures were advantageously fabricated by a process known as molecular beam epitaxy (MBE) by which smooth surfaces and extremely sharp boundaries at the interface of two closely lattice matched semiconductors can be produced with a minimum number of defects. The technique of molecular beam epitaxy is well known and has been described in, for example, L. L. Chang, et al., "Structures Grown By Molecular Beam Epitaxy", J. Vac. Sci. Technology, Vol. 10, No. 5, p. 655, September/October, 1973.
Another known fabrication technique involves the process of metallo-organic chemical vapor deposition (MOCVD) as described by N. Holonyak, Journal of Applied Physics, 1978, Vol. 49, p. 5392.
Another multi-layered semiconductor
heterostructure having potential wells created between layers is disclosed in K. Hess et al. U.S. Pat. No. 4,257,055, issued Mar. 17, 1981. The Hess et al. device comprises an inner layer which exhibits high charge carrier mobility and a relatively narrow band gap characteristic while the outer sandwich layers exhibit low charge carrier mobilities and a larger band gap characteristic. In operation, under quiescent conditions, the charge carriers of the outer sandwich layers reside in the inner layer due to the potential well created by the band gap difference between layers. The application of an appropriate electric field to the central layer, aligned with the interface between the layers, causes a very rapid transfer of electrons residing therein to the outer sandwich layers, with the resulting transfer providing a negative resistance characteristic.
U.S. Pat. No. 4,503,477, issued Mar. 5, 1985 to G. J. Iafrate, T. A. Aucoin and David K. Ferry, discloses a superlattice semiconductor device consisting of a plurality of multi-dimensional charge carrier confinement regions of semiconductor material exhibiting a low band gap which is laterally located in a single planar layer of semiconductor material exhibiting a high band gap and wherein the confinement regions have sizes and mutual separation substantially equal to or less than the appropriate deBroglie wavelength. This device comprised a thin film of semiconductor material selected from group II-VI or III-V compounds or silicon having a plurality of laterally located, cylindrically shaped period regions or wells formed therein which are adapted to act as quantum well confinement regions for electrons. These regions are GaAs islands embedded in a GaAlAs matrix. While the device of Iafrate et al. provided an advance in the art, the periodic confinement regions were formed by the cumbersome prior art technique of applying high resolution lithography and selected anisotropic etching to write and etch a pattern of cylindrical cavities in the planar surface layer, followed by selective area epitaxial growth whereby the cavities are back filled with a semiconductor material having a smaller band gap to form a periodic arrangement of rows and columns of cylindrical wells. This technique does not presently lend itself to practical commercial production, and hence this device has not met with commercial success. Thus far, the prior art has failed to provide a multi-dimensional lateral superlattice device providing quantum well transport effects which can be readily fabricated and exhibit strong negative differential conductivity which may be attributed to the onset of Bloch oscillations.
Bloch oscillations are the cylindrical movement in real space due to bragg scattering of the electrons as they traverse the reduced zones formed by the two-dimensional superlattice created by the grid. Electrons undergoing Bloch oscillations do not contribute to total current transport, so that as more electrons Bloch oscillate, current decreases. This leads to a negative differential conductivity (NDC) effect.
The existence of Bloch oscillations has long been a subject of debate. Until now, there have been no reports of a direct observation of such a phenomenon. It is generally believed that devices specially designed to create Bloch oscillations can prove valuable in the study of superlattices and that these devices would exhibit negative differential conductivity (NDC). Such devices would permit the emission of electromagnetic radiation, and the oscillation frequency would be tunable through variations in the electric field along the channel. It is conceivable that frequencies as high as tens of terahertz could be generated in this manner.
The present invention meets this need by providing novel HEMTs which exhibit strong negative differential conductivity at ambient temperature, as well as methods of fabricating such devices without the need for growing the quantum wells epitaxially as required by the prior art. In addition, the quantum well height can be adjusted by changing the voltage applied to the gate grid, unlike the epitaxially grown devices.
One such device was briefly described in our prior publication, G. Bernstein and D. K. Ferry, "Fabrication of Ultra-Short Gate MESFETs and BlochFETS by Electron Beam Lithography", "Superlattices and Microstructures", Vol. 2, No. 4, pp 373-376, 1986. However, the HEMT described therein, designated as a BlochFET because the strong negative differential conductivity is believed to be attributable to Bloch oscillations, exhibited strong negative differential conductivity only at extremely low temperatures, 4.2 K. At higher temperatures, 77K and above, the device functioned as a conventional HEMT.
This early work was also reported in G. Bernstein and D. K. Ferry, "Negative differential conductivity in lateral surface superstructures", J. Vac. Sci. Technol. B. 5(4), Jul/Aug 1987, pp 964-966; and G. Bernstein and D. K. Ferry, "Observation of Negative Differential Conductivity in a FET with Structured Gate", Z. Phys. B-Condensed Matter 67, 449-452 (1987).